rameters influencing an externally shear-strengthened beam are presented at Figure 2.16.
The parameters are grouped according to their types to:
• beam dimensions
• strengthening schemes
• FRP dimensions and characteristics.
as discussed below.
2.5.1 B e a m Dimensions
• An increase of the steel shear reinforcement is reported to reduce the FRP shear con- tribution [Uji, 1992; Sato et al., 1996; Taerwe et al., 1997; Aedy et al., 1999; Swamy et al., 1999; Deniaud and Cheng, 2000; Li et al., 2001; Neto et al., 2001; Wong, 2001;
Chaallal et al., 2002; Pellegrino and Modena, 2002; Deniaud and Cheng, 2003; Lee, 2003; Li et al., 2003; Bousselham and Chaallal, 2004a; Carolin and Taljsten, 2005;
Bousselham and Chaallal, 2006b,a; Elyasian et al., 2006; Pellegrino and Modena, 2006; Grande et al., 2007; Leung et al., 2007]. Malek and Saadatmanesh [1998a]
conducted a model study using the FEA package ABAQUS to simulate the be- haviour of shear-strengthened beams. From this model, the authors reported that the FRP sheets shear contribution does not depend on the presence of steel stirrups.
• The influence of the concrete strength to the effective strain of the FRP plies was only studied by Deniaud and Cheng [2000]. They reported that the effective strain of the FRP is observed to increase proportionally with the increase of the concrete strength.
Khalifa and Nanni [2000], Triantafillou and Antonopoulos [2000] and Elyasian et al.
[2006] confirmed the same trend.
• Khalifa and Nanni [2002] observed from their experimental works that the shear span to effective depth ratio (a/d) influences the FRP shear contribution, which appears to decrease with the decrease of the ratio. This is opposed to what was reported by Cao et al. [2005], who stated that the shear capacity increases with the decrease of shear span/depth ratio. From the tests conducted by Mitsui et al. [1998], it is
mentioned that the shear contribution of the FRP plies has no distinct relation with this ratio. A literature review of the parameters influencing the shear behaviour of strengthened beams was carried out by Bousselham and Chaallal [2004a]. In their study, three zones of a/d were identified, such as: (a) the zone corresponding to a/d less than 2.5, where failure is predominately by fracture; (b) the zone corresponding to (a/d) greater than 3.2, where failure is by debonding; and (c) a transition zone is ranged between the above values, where the failure in this case can be either by debonding or by fracture.
• The best corner radius is found to minimize the effect of stress concentration pro- duced at the sharp corners is when the corner radius about 0.25 of the width of the section [Yang et al., 2001]. Radii of 35 mm,13 mm and 15 mm minimum were proposed by the ISIS [2001], ACI [2002] and the BS [2000], respectively.
• The size effect of shear-strengthened beams attracted a few researchers for investi- gation, particularly for specimens that have an effective depth greater than approx- imately 300mm. The gain in shear strength tends to decrease with increasing the effective depth [Deniaud and Cheng, 2000; Qu et al., 2005; Leung et al., 2007]. With increase of depth size, the failure mode changes from fracture to debonding of the FRP sheets [Bousselham and Chaallal, 2004a]. By contrast, [Triantafillou, 1998] and [Hassan et al., 2007] linked the size effect in the members with the bonded surface area. The deeper the beam, the more the FRP contribution to the shear resistance.
• [Li et al., 2001] found that the FRP sheets shear contribution is greatly influenced by the area of the longitudinal steel. The effective strain of FRP sheets increases when the area of the longitudinal steel decreases. The opposite of this was reported by Bousselham and Chaallal [2004a, 2006b]; Hassan et al. [2007], The effective strain of FRP sheets is not proportional to the longitudinal steel.
2.5.2 Strengthening Schemes
• When there is no access to full wrapping of the beams, all the researchers confirmed that the repair of beams with U-jacketing is the best solution.
• No difference was recorded in the shear capacity when strips or continuous sheets were used [Al-Sulaimani et al., 1994; Malek and Saadatmanesh, 1998a]. Khalifa and
2.5. PARAMETERS INFLUENCING SHEAR-STRENGTHENED BEAMS
Nanni [2000] noticed that sometimes FRP strips give higher shear capacity than continuous sheets. The use of continuous sheets increases the shear capacity of the strengthened beams more than using FRP strips [Taerwe et al., 1997; Khalifa and Nanni, 2002; Abdel-Jaber et al., 2003; Cao et al., 2005; Monti and Liotta, 2007].
This was attributed to the smaller bond area of FRP strips.
• Strengthened beams with inclined fibres were found to be more effective in resisting crack propagation than vertical strips [Uji, 1992; Chajes et al., 1995; Norris et al., 1997; Chaallal et al., 1998a; Deniaud, 2000; Deniaud and Cheng, 2001a; Neto et al., 2001; Diagana et al., 2003; Li et al., 2003; Zhang and Hsu, 2005; Monti and Liotta, 2007]. The tests conducted by Carolin and Taljsten [2005] showed no difference in shear capacity between vertical and inclined fibre directions. The vertical orientation of the FRPs provides higher strength gain and a more ductile failure than the inclined FRPs [Elyasian et al., 2006].
• Biaxial or triaxial FRP plates provide the beam with more ductile failure than unidirectional plates and no increase in load carrying capacity was observed [Norris et al., 1997; Deniaud and Cheng, 2003]. The biaxial FRP plates have no influence on the shear strength [Alexander and Cheng, 1996; Khalifa and Nanni, 2000]. Khalifa and Nanni [2002] and Chaallal et al. [2002] observed a considerable increase in the beams strengthened with biaxial sheets.
• Strengthening with double FRP plies increases the load carrying capacity but re- duces the ductility of the section [Neto et al., 2001; Chaallal et al., 2002; Pellegrino and Modena, 2002; Carolin and Taljsten, 2005; Bousselham and Chaallal, 2006b;
Pellegrino and Modena, 2006].
• Taerwe et al. [1997] tried to strengthen beams with U-jackets around the compres- sion face of the beam instead of the tension face. He found that this method of strengthening led to debonding unlike the usual method, therefore reducing the FRP shear contribution. Most of the researchers who studied horizontally oriented fibres concluded the same result [Alexander and Cheng, 1996; Carolin and Taljsten, 2005; Zhang and Hsu, 2005]. Normally the strengthening scheme with horizontal fibres has no significant contribution to the shear carrying capacity. By contrast, Adhikary and Mutsuyoshi [2004] and Khalifa and Nanni [2002] concluded that beams with horizontally aligned bonded fibres showed enhanced shear strength.
• Taljsten and Elfgren [2000] examined different ways of attaching the FRP sheets to the concrete beams, which included hand lay-up, pre-preg with vacuum and heat and vacuum injection. The hand lay-up technique was found to be the optimum way of applying the FRP sheets in terms of applied load/deflection. There was no difference between the other ways of strengthening.
2.5.3 F R P Dimensions and Characteristics
• The contribution of the FRP plates to the shear capacity and the ductility of the beam increased almost linearly with the increase of the plate thickness [Malek and Saadatmanesh, 1998a; Li et al., 2001; Carolin and Taljsten, 2005]. The increase of the plate thickness was reported by others to reduce the shear contribution of the FRP plies [Triantafillou, 1997; Chaallal et al., 1998a; Triantafillou and Antonopoulos, 2000].
• The maximum shear contribution of the FRP composites was found when the plate stops at a point near the neutral axis of the strengthened beam [Malek and Saa- datmanesh, 1998a]. Alexander and Cheng [1996] stated a minimum length of 75mm for the FRPs in order to develop their full capacity. Li et al. [2001] proposed that the FRP strip height should be equal to three fourths of the beam height to ob- tain the optimum contribution of the FRP plies. For the U-wrap scheme studies by Abdel-Jaber et al. [2003], Adhikary and Mutsuyoshi [2004] and Zhang and Hsu [2005], the best FRP depth is when the FRP sheet is placed up to the maximum possible section depth.
• There is a threshold for the shear contribution with respect to the axial rigidity of the FRP composites, which is defined by the area times the modulus of elas- ticity expressed by the product pfrpEfrp. It was observed that the shear capacity increases linearly as the axial rigidity of the FRP increases until the value of axial rigidity reaches a OAGPa, then tends to stabilize thereafter [Triantafillou, 1997].
Bousselham and Chaallal [2004a] used a new indication called the shear rigidity in their survey of the published experimental results and they stated the same conclu- sion at a shear rigidity equal to 0.05. Teng et al. [2002] reported that the effective bond length increases linearly with an increase of yjEftj; where Ef and tf are the modulus of elasticity and the thickness of the FRP plate, respectively.